This nonprovisional application claims priority under 35 U.S.C xc2xa7119(a) on Patent Application No. 100 34 325.2 filed in Germany on Jul. 14, 2000, which is herein incorporated by reference.
1. Field of Invention
The invention relates to a tuning circuit according to the preamble to the main claim.
2. Description of the Background Art
Tuning circuits of this type for YIG oscillators are known (book by U. Rohde, xe2x80x9cMicrowave and Wireless Synthesizersxe2x80x9d, page 402 ff, spectrum analyzers FSA and FSE manufactured by Rohde and Schwarz). YIG oscillators are used as oscillators which are capable of being tuned over a wide frequency range of, for example, 4 to 12 GHz and which are synchronized via an appropriate phase-control loop to a lower reference frequency which, for example, is prespecified by a synthesizer. At the same time, great demands are made of the phase jitter of the YIG oscillator outside the controlled bandwidth; in addition, the tuning-rate and the transient oscillation in the event of large jumps in frequency are to be, as far as possible, of the same order of magnitude as in the case with conventional voltage-controlled oscillators capable of being tuned via variable-capacitance diodes.
Tuning of the oscillation frequency is effected in the case of a YIG oscillator by means of a magnetic field, whereby relatively high magnetic field strengths are required. The greater part of this field strength is generated by a main coil with iron core; fine tuning is effected with the aid of a smaller air-cored coil (also called an FM coil). By virtue of the large inductance of the main coil in conjunction with the associated stray capacitances, a YIG oscillator can be tracked via the main coil only in the narrow band. For bandwidths greater than 100 kHz, closed-loop control is effected via the air-cored coil, the capture range of which is limited by the constructional features of the YIG oscillator to about 50 MHz. In the case of a known tuning circuit for a YIG oscillator the main coil is fed with a pretuning current which is generated from the desired output frequency of the YIG oscillator which is digitally prespecified in a processor via a digital/analogue converter and an appropriately controllable current source. Synchronization is effected separately thereof via the air-cored coil as a function of the output voltage of the phase detector. A disadvantage of this known arrangement is the noise current produced by the current source of the main coil, by virtue of which the YIG oscillator is modulated with strong phase jitter outside the controlled bandwidth. With a view to improving the phase jitter, it is in fact known to connect another capacitor in parallel with the main coil, which together with the main coil forms a resonating circuit, as a result of which, however, the frequency-setting rate is reduced in the event of large jumps in frequency. Accordingly, a rapid change of frequency is only possible without a capacitor, but then the phase jitter is again inadmissibly bad.
It is therefore an object of the present invention to provide a turning circuit for a YIG oscillator, wherein despite little circuit complexity the phase jitter is low and the turning-rate is high.
This object is achieved, starting from a tuning circuit according to the preamble to the main claim, by virtue of the characterising features thereof. Advantageous further developments are evident from the subordinate claims.
According to the invention the main coil of the YIG oscillator is also fed directly by the output voltage of the phase detector. The current through the main coil results from the applied voltage divided by the impedance of the main coil. Since the reactance rises with increasing frequency, this leads to low-pass behaviour. On account of this low-pass behaviour and on account of the low output impedance of the voltage source feeding the main coil, this circuit impresses only a negligible noise current. The tuning-rate is not restricted, since a capacitor located parallel to the main coil is superfluous. Transient phenomena are also avoided.
The d.c. resistance of the main coil is strongly temperature-dependent. The interrelationship between voltage and frequency is therefore very imprecise. Therefore according to a further development of the invention as specified in the dependent claims not only an additional pretuning voltage for the main coil is proposed but also an appropriate limitation of the maximal and minimal main-coil currents via the measurement of the drop in voltage arising at a series resistor of the main coil.
In contrast with the state of the art, the main coil is no longer fed from a current source but from a voltage source. It is a characteristic of a voltage source that the latter has a low output impedance of practically zero and consequently impresses only a negligible noise current upon the main coil.
The invention is elucidated in greater detail in the following on the basis of schematic drawings with reference to embodiment examples.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.